The steady progress in semiconductor technology over the last several decades has been characterized largely by dramatic increases in circuit density. These increases have been made possible by corresponding improvements in semiconductor processing techniques for forming ever smaller geometric structures on semiconductor dies. These process improvements, however, involve much more than simple scaling of semiconductor and interconnect structures.
As semiconductor devices are scaled down in size, various practical physical limitations of the circuit elements and their interconnection structures are reached. These limits frequently require the semiconductor device designer to re-think the entire approach to the design of semiconductor circuit elements (e.g., transistors) in order to take full advantage of the smaller geometries made possible by improved processing techniques. This periodic “re-thinking” of semiconductor device design produces an ever-evolving landscape of transistor designs and circuit topologies.
Within any one generation of semiconductor device technology, it is frequently desirable to reduce the silicon “footprint” of a device and/or circuit. Io accomplish this without making fundamental changes to the design, it is common to create high-aspect ratio structures. For example, in order to shrink the width of a conductive line without reducing its current carrying capacity, it is necessary to make the conductive line taller. This requires a process capable of producing a high-aspect ratio structure.
High aspect-ratio lines are often formed by creating high aspect ratio trenches (skinny, deep trenches), then filling them with conductive material (e.g., polysilicon or metal). One well-known challenge in forming such structures is the problem of “pinch-off”. In performing deep etches, a “crust” or “cusp” forms on the sidewalls of the trench being etched. If the trench is narrow enough (high aspect ratio), then as the reaction proceeds this crust can become thick enough to effectively close over or “pinch off” the top of the trench, thereby preventing etching reactants from migrating into the lower portions of the trench. This “pinch off” effect has a limiting effect on the aspect ratio of trench etches.
A similar pinch-off effect can occur when depositing material (e.g., conductive materials) into a trench. As the deposition proceeds, the deposited material accumulates disproportionately along the upper portions of the trench. If the trench is narrow enough, the material may close over or “pinch off” before the trench is completely filled, thereby forming a void in the deposited material. This has a limiting effect on the aspect ratio of deposited materials. In the case of conductors, such a void results in high resistance, which is antithetical to the purpose of creating a high aspect-ratio conductor in the first place.
Evidently, in order to continue shrinking semiconductor device footprints, it would be beneficial to have a method for producing high aspect ratio structures that reduces or eliminates the problem of “pinch off”.